A communication system is operable to communicate information between a transmitting station and a receiving station by way of a communication channel. A radio communication system is a communication system in which the communication channel by which information is communicated between the transmitting and receiving stations is formed upon a portion of the electromagnetic spectrum. A cellular communication system is exemplary of a multi-user, radio communication system.
The portion of the electromagnetic spectrum allocated to a radio communication system is typically bandwidth-limited. That is to say, only a limited spectrum portion, referred to as "bandwidth", is permitted to be used by a particular radio communication system. All of the radio channels available for use in the system must be defined within the allocated bandwidth. The capacity of the radio communication system is sometimes limited by the bandwidth allocated to the system.
Efficient utilization of the bandwidth allocated to the radio communication system is therefore required to ensure that the communication capacity of the radio communication system is maximized. Manners by which to utilize more efficiently the bandwidth allocated to a radio communication system permits greater numbers of channels to be defined within the allocated bandwidth.
Advancements in technologies have facilitated more efficient use of the bandwidth allocated for radio communication systems. Radio communication systems in which advanced communication technologies have been implemented permit increased numbers of communication channels to be defined within the allocated bandwidth, thereby to increase the effective communication capacity of the system.
For instance, in some radio communication systems, digital modulation techniques have been implemented to increase the effective capacity of the communication system. When digital modulation techniques are utilized in a radio communication system, a lessened amount of frequency spectrum is required for the communication of information between sending and receiving stations operable therein. When a digital modulation technique is utilized, a single carrier can be divided into a plurality of channels so that a single carrier can be used to transmit information between a plurality of sending and receiving stations.
In a radio communication system utilizing a digital modulation technique, as well as other modulation techniques, information is modulated upon a carrier of a frequency to form a modulated signal centered at, or about, the carrier frequency upon which the radio channel is defined. The carrier upon which the information is modulated must be of frequency stability characteristics good enough to ensure that the modulated signal does not drift from the carrier upon which the channel is defined. Otherwise, if the carrier signal upon which the information is modulated is not of an adequate frequency stability, a modulated signal transmitted by a transmitting station might drift away from a designated channel and interfere with ongoing communications on another channel.
Attempts are made, therefore, to ensure that the carrier upon which information is modulated is of acceptable frequency stability characteristics. Phase-locked-loop (PLL) circuits, for instance, oftentimes form portions of a transmitting station. A phase-locked-loop (PLL) circuit typically includes a voltage-controlled oscillator (VCO) having an output oscillating signal of a frequency which is related to an input reference signal. A signal related to the output oscillating signal generated by the VCO is compared with the input reference frequency. Responsive to phase differences between such signals, a voltage is applied to the VCO either to increase, or to decrease, the frequency of the output, oscillating signal.
A frequency divider is typically positioned in a feedback loop of the PLL circuit. The frequency divider divides the output oscillating signal by an integer value to form a frequency-divided signal. The frequency-divided signal forms the signal which is compared with the input reference signal. In some PLL circuits, the frequency divider can only divide the output oscillating signal by set, stepped amounts, e.g., integer amounts. And, the PLL circuit is able merely to generate output oscillating signals of set, stepped frequencies as the output frequency is equal to the reference frequency multiplied by the division factor of the frequency divider. The resolution of the output oscillating signals is therefore limited.
To improve resolution, some PLL circuits utilize fractional n synthesis by including a .SIGMA..DELTA. modulator. The division factor by which the frequency divider divides the output oscillating signal is determined by a signal generated by a .SIGMA..DELTA. modulator. Such an arrangement is sometimes referred to as a .SIGMA..DELTA.-controlled PLL circuit. Use of a .SIGMA..DELTA. modulator is advantageous as both higher-frequency resolution and higher bandwidth of the PLL is permitted. When a .SIGMA..DELTA.-controlled PLL circuit is formed, both cost and space efficiencies are provided. Also, such an arrangement permits continuous phase modulated signals to be generated. And, direct and digital control over modulation, as well as channel selection, can be provided in apparatus in which such an arrangement is embodied. U.S. Pat. No. 5,055,802, for instance, discloses a frequency synthesizer which utilizes a .SIGMA..DELTA. modulator.
However, particularly when an input signal applied to the .SIGMA..DELTA. modulator is of a constant frequency value, the modulator is susceptible to enter into what is referred to as a "limit cycle." The division-factor control signal might then begin to repeat itself. When such a signal is applied to the frequency divider of the PLL circuit, the output oscillating signal might exhibit unwanted tones. Such tones deleteriously effect operation of a sending, or receiving, station of which the .SIGMA..DELTA. modulator-controlled PLL circuit forms a portion.
While some manners have been developed to reduce problems associated with the generation of tones caused by repetitive behavior of the .SIGMA..DELTA. modulator, such manners are expensive and difficult to implement. For instance, feed-forwarding of an analog error signal to the PLL circuit to cancel the error caused by the repetitive behavior of the .SIGMA..DELTA. modulator is sometimes provided. Difficulty and expense results as matching of RF hardware is required and, in any event, component value uncertainty limits the successful implementation of such a solution.
A manner by which better to assure acceptable operation of a .SIGMA..DELTA. modulator of a .SIGMA..DELTA. modulator-controlled PLL circuit would therefore be advantageous.
It is in light of this background information related to PLL circuits that the significant improvements of the present invention have evolved.